The parent application identified above disclosed an electronic circuit that alters, in a passive and dynamic manner, the resonant frequency tuning of a radio transceiver's antenna circuit, comprising an inductor and capacitor. More specifically, the function of the antenna circuit disclosed therein is the radio transceiver's transmitting and receiving transducer that interacts with a passive identification transponder, generally known as a “half-duplex”, or “HDX” type transponder. In transponders of the HDX variety, the radio transceiver transmits an activation signal to the transponder via its resonant antenna circuit during a first interval (the transmit interval), and receives the transponder's data signal via its resonant antenna circuit during a second interval (the receive interval) when the radio transceiver's transmit signal is squelched.
In the particular embodiments described in the parent application, the radio transceiver transmits an activation signal comprising a radio frequency of 134.2 KHz, and receives frequency shift keyed (FSK) modulation comprising the frequencies 134.2 KHz and 124.2 KHz to convey binary data from the HDX transponder. The parent application describes in detail how the radio transceiver operates more efficiently when the resonant antenna circuit is tuned to 134.2 KHz during the transmit interval, and tuned nominally to 129.2 KHz (mid-point between the HDX transponder's FSK data frequencies) during the receive interval.
As shown in FIG. 1A (corresponding to FIG. 4A of the parent application), the tuning of the radio transceiver's resonant antenna circuit is shifted from 134.2 KHz during the transmit interval to 129.2 KHz during the receive interval, through the use of three passive components, diodes D1 [101a], D2 [102a], and inductor LT [103a]. During the transmit interval, the transmit signal F [106a] is conducted through diodes D1 [101a] and D2 [102a], such that inductor LT [103a] is electrically bypassed, or shorted, and the antenna's resonant frequency tuning is determined by the capacitor C [104a] and antenna inductor L [105a]. Subsequently, during the receive interval, when there is no transmit signal F [106a] present, diodes D1 [101a] and D2 [102a] become open circuits such that inductor LT [103a] is electrically inserted in the resonant circuit. Thus, the antenna's resonant frequency tuning is determined by the capacitor C [104a] and the series combination of antenna inductor L [105a] and inductor LT [103a].
As is well understood by those of ordinary skill in the art, the resonant frequency of the inductor L [105b]/capacitor C [104b] tuned circuit, as shown in FIG. 1B, is:FRT=1/[2π√LC]Thus, as is disclosed in detail in the parent application, the resonant frequency of the antenna circuit is defined by the above equation during the transmit interval when inductor LT [103a] is electrically bypassed, and becomesFRR=1/[2π√(L+LT)C]during the receive interval when inductor LT [103c] is electrically inserted in the resonant circuit, as shown in FIG. 1C. Consequently, by selecting an appropriate inductance value for inductor LT [103c], the resonant frequency tuning of the antenna circuit can be shifted in a passive and dynamic manner between FRT=134.2 KHz during the transmit interval, and FRR=129.2 KHz during the receive interval.
As was further disclosed in the parent application, the advantage of the electrical frequency shifting circuit comprising diodes D1 [101a], D2 [102a], and inductor LT [103a] is its simplicity, low cost, and high reliability. However, there exist alternate embodiments that accomplish this frequency shifting technique, and although not passive, are equally effective in achieving an equivalent result. These alternate methods comprise the subject matter of the present continuation-in-part invention.